Digital compensation method and apparatus using image-to-image distortion map relating reference mark grids

Abstract

A digital image processing method. The method includes printing a first set of reference marks on one side of a substrate with a first print engine; printing a second set of reference marks on the same side of the substrate as the first set of reference marks with a second print engine; sensing both sets of reference marks on the substrate with an image sensing unit and generating a digital image of the reference marks; performing image analysis on the digital image to obtain an image-to-image distortion map where the image-to-image distortion map is a local measure of difference between the first set of reference marks and the second set of reference marks; and generating a compensated customer image by using the image-to-image distortion map to reduce registration errors when using the first and second print engines.

1. A method comprising:
 printing a first set of reference marks on one side of a substrate with a first print engine; 
 printing a second set of reference marks on the same side of the substrate as the first set of reference marks with a second print engine; 
 sensing both sets of reference marks on the substrate with an image sensing unit and generating a digital image of the reference marks; 
 performing image analysis on the digital image to obtain an image-to-image distortion map where the image-to-image distortion map is a local measure of difference between the first set of reference marks and the second set of reference marks, wherein the image-to-image distortion map comprises an array of distances relating a grid formed by the second set of reference marks to a grid formed by the first set of registration marks; and 
 generating a compensated customer image by using the image-to-image distortion map to reduce registration errors when using the first and second print engines. 
 
     
     
       2. The method of  claim 1 , wherein, prior to obtaining the image-to image distortion map, the image analysis includes determining global errors between the reference marks and where the global errors were not repeatable over multiple sets of prints, eliminating one of global lateral offset between the first set of reference marks and the second set of reference marks, global rotation between the first set of reference marks and the second set of reference marks, and global lateral offset and global rotation between the first set of reference marks and the second set of reference marks. 
     
     
       3. The method of  claim 1 , wherein, prior to obtaining the image-to image distortion map, the image analysis includes determining global errors between the reference marks and where the global errors were repeatable over multiple sets of prints, including the global errors in the distortion to be corrected in future customer prints. 
     
     
       4. The method of  claim 1 , wherein, prior to obtaining the image-to image distortion map, the image analysis includes determining global errors between the reference marks and where one or more global errors is more significant than a set of local displacements, representing the distortion map used for compensation by both global parameters and local displacements. 
     
     
       5. The method of  claim 1 , wherein, prior to obtaining the image-to image distortion map, the image analysis includes determining global errors between the reference marks and where the global errors and a first set of local displacements are both repeatable, representing the distortion map used for compensation by both global parameters and local displacements or converting the global errors to a second set of local displacements and combined with the first set of local displacements. 
     
     
       6. The method of  claim 1 , wherein each set of reference marks comprises a plurality of one of dots in a written grid pattern and lines in a written grid pattern. 
     
     
       7. The method of  claim 6 , wherein each written grid pattern represents a different colorant. 
     
     
       8. The method of  claim 7 , wherein the first written grid pattern is black and the second grid pattern is one of cyan, magenta, yellow, black, orange, violet, red, green, light cyan, light magenta, gray, and dark yellow. 
     
     
       9. The method of  claim 1 , wherein the image sensing unit comprises a full width array. 
     
     
       10. The method of  claim 1 , wherein each of the print engines comprises a xerographic printer. 
     
     
       11. An apparatus comprising:
 a first print engine that prints a first set of reference marks on one side of a substrate; 
 a second print engine that prints a second set of reference marks on the same side of the substrate; 
 an image sensing unit operative to sense both sets of reference marks on the substrate to generate a digital image of the reference marks; 
 a distortion determining unit operative to determine distortion between the print engines based on analysis of the digital image of the reference marks, wherein the distortion is represented by an image-to-image distortion map comprising an array of distances relating to a grid formed by the second set of reference marks to a grid formed by the first set of reference marks; and 
 a distortion compensation unit operative to generate a digital image that has been compensated to reduce registration errors between images produced by the first print engine and second print engine through the use of the distortion determined from the reference marks from the two print engines. 
 
     
     
       12. The apparatus of  claim 11 , wherein the image analysis includes estimating the relative registration errors in the second set of reference marks with respect to the first set of reference marks. 
     
     
       13. The apparatus of  claim 11 , wherein each set of reference marks comprises one of a plurality of dots in a grid pattern and a plurality of lines in a grid pattern. 
     
     
       14. The apparatus of  claim 13 , wherein each grid pattern represents a different colorant. 
     
     
       15. The apparatus of  claim 13 , wherein the first grid pattern is black and the second grid pattern is one of cyan, magenta, yellow, black, orange, violet, red, green, light cyan, light magenta, gray, and dark yellow. 
     
     
       16. The apparatus of  claim 11 , wherein the image sensing unit comprises a full width array. 
     
     
       17. The apparatus of  claim 11 , wherein each of the print engines comprises a xerographic printer. 
     
     
       18. The apparatus of  claim 11 , wherein the registration errors comprise at least one of translational, rotational and magnification errors. 
     
     
       19. The apparatus of  claim 11 , wherein the distortion compensation unit is operatively connected to the second print engine. 
     
     
       20. A non-transitory computer readable medium
 storing instructions that, when executed by a computer, cause the computer to perform a method comprising: 
 printing a first set of reference marks on one side of a substrate with a first print engine; 
 printing a second set of reference marks on the same side of the substrate as the first set of reference marks with a second print engine; 
 sensing both sets of reference marks on the substrate with an image sensing unit and generating a digital image of the reference marks; 
 performing image analysis on the digital image to obtain an image-to-image distortion map where the image-to-image distortion map a is local measure of difference between the first set of registration marks and the second set of registration marks, wherein the image-to-image distortion map consists of an array of distances relating a grid formed by the second set of reference marks to a grid formed by the first set of registration marks; and 
 generating a compensated digital image by using the image-to-image distortion map to reduce registration errors when using the first and second print engines. 
 
     
     
       21. The computer program product of  claim 20 , wherein, prior to obtaining the image-to image distortion map, the image analysis includes eliminating one of global lateral offset between the first set of reference marks and the second set of reference marks, global rotation between the first set of reference marks and the second set of reference marks, and global lateral offset and global rotation between the first set of reference marks and the second set of reference marks.